Guppies can breed and hold sperm for quite some time. They can give birth every few weeks until the last sperm has been fertilized. This is why there is an old wives tail about fish changing their sex to be able to breed. They do not CHANGE their sex, but can hold sperm long enough to produce many separate babies at different ages. The key, is to separate the mother from the babies so she does not continue to breed with sons from her previous breeding.